1. Field of the Invention
The present invention relates to a GPS reception apparatus configured to calculate current position based on satellite information on a plurality of satellites obtained from GPS signals, and a positional calculation method for the GPS reception apparatus.
2. Description of the Related Art
Various Global Positioning System (GPS) reception apparatuses are commonly used for different purposes; GPS reception apparatuses measure the current position by utilizing GPS to receive electromagnetic signals transmitted by a plurality of satellites. The GPS reception apparatus acquires at least four GPS satellites based on the general positions of the respective GPS satellites indicated by almanac information pre-obtained from the GPS satellites. The GPS reception apparatus then utilizes ephemeris information contained in a GPS signal received from each of the GPS satellites, that is, navigation data (satellite information), to accurately calculate positional information on a user (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-61336).
On the other hand, the navigation data includes time information from a precise clock provided in each satellite, that is, time-of-week (TOW) that is a numerical value in units of 6 seconds counted from 00:00:00 on every Sunday. Thus, the accurate current time can be obtained from the time information (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2007-263598).
If the accurate current time is obtained from the time information included in the navigation data, then since a time reference for GPS (GPS time) is coordinated universal time (UTC), the time obtained from the time information needs to be corrected in accordance with a time difference from UTC, which is a standard time adopted in a district (a country or the like) to which the current position (the place where the user is present) belongs, in order to obtain the correct current time.
However, if the GPS reception apparatus is incorporated in a portable electronic machine, for example, a watch, when the user is in an alley surround by buildings, in a place surrounded by trees, or in a house or the user is moving with the GPS reception apparatus, then a number of satellites required to calculate the current position cannot be acquired. Even if a required number of satellites can be acquired, reception sensitivity is low, and ephemeris information often fails to be obtained from the GPS signal. In this case, even though the time information can be obtained, the current position cannot be calculated. As a result, the correct current time cannot be obtained.
Furthermore, to avoid the above-described circumstances, the operation of receiving GPS signals need to be repeated until ephemeris information is obtained. However, in this case, time setting involves high power consumption, thus reducing the continuous operation time of the apparatus. In particular, disadvantageously, if small batteries such as coin-shaped ones are used, the continuous operation time decreases significantly.